The relevant velocity that describes transport phenomena in a porous
medium is the pore velocity. For this reason, one needs not only to
describe the variability of transmissivity, which fully determines the
Darcy velocity field for given source terms and boundary conditions,
but also any variability of the pore volume. We demonstrate that
hydraulically equivalent media with exactly the same transmissivity
field can produce dramatic differences in the displacement of a solute
if they have different pore volume distributions. In particular, we
demonstrate that correlation between pore volume and transmissivity
leads to a much smoother and more homogeneous solute distribution. This
was observed in a laboratory experiment performed in artificial
fractures made of two plexiglass plates into which a space-dependent
aperture distribution was milled. Using visualization by a light
transmission technique, we observe that the solute behaviour is much
smoother and more regular after the fractures are filled with glass
powder, which plays the role of a homogeneous fault gouge material.
This is due to a perfect correlation between pore volume and
transmissivity that causes pore velocity to be not directly dependent
on the transmissivity, but only indirectly through the hydraulic
gradient, which is a much smoother function due to the diffusive
behaviour of the flow equation acting as a filter. This smoothing
property of the pore volume-transmissivity correlation is also
supported by numerical simulations of tracer tests in a dipole flow
field. Three different conceptual models are used: an empty fracture, a
rough-walled fracture filled with a homogeneous material and a
parallel-plate fracture with a heterogeneous fault gouge. All three
models are hydraulically equivalent, yet they have a different pore
volume distribution. Even if piezometric heads and specific flow rates
are exactly the same at any point of the domain, the transport process
differs dramatically. These differences make it important to
discriminate in situ among different conceptual models in order to
simulate correctly the transport phenomena. For this reason, we study
the solute breakthrough and recovery curves at the extraction wells.
Our numerical case studies show that discrimination on the basis of
such data might be impossible except under very favourable conditions,
i.e. the integral scale of the transmissivity field has to be known and
small compared to the dipole size. If the latter conditions are
satisfied, discrimination between the rough-walled fracture filled with
a homogeneous material and the other two models becomes possible,
whereas the parallel-plate fracture with a heterogeneous fault gouge
and the empty fracture still show identifiability problems. The latter
may be solved by inspection of aperture and pressure testing. (C) 2003
Elsevier Science B.V. All rights reserved.